The Earth has a crust under the
oceans and the land that we live on. This crust is made of
massive areas of flat rock called tectonic plates, which float on
the Earth’s mantle, the inside layer of the Earth.

Where these plates meet is called a fault. The picture above shows the Earth’s tectonic plates and how they fit
together like a jigsaw.

When the plates move slowly together, this movement forces energy through
the crust to the earth’s surface.

The energy causes the earth to tremble and shake. These tremors are called
an earthquake.

Any shaking of the Earth’s crust is an earthquake. Some earthquakes are very small and can only be detected by sensitive
instruments. Other earthquakes are huge and cause massive
destruction and loss of life.

The strength of earthquakes are measured by an instrument called a seismograph. This word is
taken from the Ancient Greek words, seismos, which means
earthquake, and graphein, which means writing.The study of earthquakes is
called seismography.

Instruments for investigating earthquakes have existed from very early
times. A Chinese scientist called Choko invented an instrument
in AD 136 which recorded earthquakes and the direction from which the shock came.

Modern seismographs can record the strength of an earthquake very
accurately. Seismographs are also known as seismometers (meaning
earthquake-measurer).

The way in which an earthquake develops is rather like what you see if you
throw a stone into water. When the stone hits the water, a
series of waves move out in circles. We call these concentric waves, meaning they are moving out from the centre in
circles.

An earthquake works in the same way. There is a sudden movement between tectonic plates and concentric shock
waves move outwards from that movement. The centre point of the
earthquake is called the epicentre.

Earthquakes are measured on a scale called the Richter scale. The numbers on the Richter scale measure factors of 10, which means that an
earthquake which measures 5 on the Richter scale is 10 times bigger than one that measures
4.

·Charles Richter developed the Richter scale for measuring earthquakes in
1935.

·Earthquakes measuring 2 and under on the Richter scale can only be detected
by seismometers. We cannot feel these small tremors at
all. These very small earthquakes are called microquakes and are happening all the time.

·Earthquakes measuring between 2 and 6 on the Richter scale are classed as
moderate. They may cause minor damage.

·Earthquakes measuring 7 and above on the Richter scale can cause terrible
damage and be disastrous.

·The largest recorded earthquake was in Chile in 1960 and measured 9.5 on
the Richter scale.

The Earth is shaken by at least one powerful earthquake every
day. Fortunately most of these take place in areas far from
towns and cities. Many of them take place under the
sea.

Undersea earthquakes are not always harmless, however. An earthquake under the sea can cause a terrible tidal wave called a
tsunami, which can sweep onto the land destroying everything in
its path.

·Indian Ocean tsunami, 2004. In December 2004 an undersea earthquake with an
epicentre off the west coast of Indonesia caused a tsunami with waves of up to 95 feet. Over 230,000 people
were killed in 14 different countries.

·Japanese tsunami, 2011. In March 2011 there was an earthquake under the sea
just off the Japanese coast. The earthquake measured 8.9 on the
Richter scale and created a giant tsunami. Both the shock waves
of the earthquake and the tsunami caused terrible damage in Japan and killed more than 20,000 people.